1. Field of the Invention
This invention relates to an aperture device of a lens capable of photographing an object at close proximity by the use of a flash unit, particularly an electric flash unit.
2. Description of the Prior Art
When an electronic flash unit is used as an object illuminating light source, if the spacing between the optical axis of the phototaking lens of the camera and the flash unit comes near the object, the light distributing characteristic of the flash unit exerts a very strong influence upon the illumination of the object and thus, it has been impossible to effect uniform illumination. As a technique of solving this problem, it has been adopted to mount a ring-shaped xenon tube of the flash unit around the end of the lens and thereby illuminate an object at proximity.
Generally, where auxiliary illumination photography is to be effected with an object illuminated by an electronic flash unit, such auxiliary light photography is carried by varying the aperture value of the camera in accordance with the object distance so that the total quantity of emitted light inherent to the flash unit, namely, the quantity of emitted light usually called the guide number for ASA100, becomes equal to the product of the distance to the object and the aperture opening value of the camera. However, when the position of the camera is very close to the object, the distance to the object is very small and therefore, a proper exposure cannot be obtained unless an aperture opening equal to or less than the minimum aperture value that can be adjusted by the aperture device of the camera is provided. Accordingly, with such system, proximity photography has been impossible.